Amaranthus caudatus agglutinin (ACA) is a kind of phytoagglutinin (plant lectin) present in the seeds of Amaranthus caudatus having high nutritive value originated in South America continent [Vietmeyer, N. et al (1986), Science 232:1379–1384] and is mainly as a storage protein providing nutrition during the seed germination. ACA is abundantly synthesized in seeds during the formation of endosperm, but little in vegetative organs. The amino acid sequence of ACA protein has been reported in GenBank (Accession No. g2781234). Rahbe Y. et al carried out in vitro feeding pea aphids (Acyrthosiphon pisum) with different purified plant lectin proteins and the results showed that ACA gave the very high inhibition or toxic effect for tested aphids [Rahbe Y, Sauvion N, Febvay G et al 1995, Entomol Exp Appl, 76:143–155]. It is reported that this agglutinin plays an important role in specific recognition of tumor cells, histochemical identification and early diagnosis of tumors [Boland, C. et al, (1991), Cancer Res. 51:657–665]. As the function of plant agglutinins being further elucidated and potential application values thereof in medicine and biology being understood, it appears necessary to clone the ACA gene for studying and applying the agglutinin. Though the aphid-resistant activity of ACA has been revealed in the insect-resistant test in vitro as described, this activity has not been verified in transgenic plants expressing ACA gene. Moreover, ACA protein as a protein being rich in essential amino acid residues may play an important role in improving quality of crop. High level expression of ACA protein obtained in vitro conveniently could be a sufficient supply of this protein for identification of some tumor diseases.